Nanophotovoltaic devices

ABSTRACT

The present invention provides nanophotovoltaic devices having sizes in a range of about 50 nm to about 5 microns, and method of their fabrication. In some embodiments, the nanophotovoltaic device includes a semiconductor core, e.g., formed of silicon, sandwiched between two metallic layers, one of which forms a Schottky barrier junction with the semiconductor core and the other forms an ohmic contact therewith. In other embodiment, the nanophotovoltaic device includes a semiconductor core comprising a p-n junction that is sandwiched between two metallic layers forming ohmic contacts with the core.

RELATED APPLICATIONS

The present application is a divisional application of U.S. patentapplication Ser. No. 11/002,850, filed on Nov. 30, 2004, which is hereinincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention is directed generally to nanometer-sized quantumstructures, and more particularly to such structures that can beselectively activated to perform a desired function, for example, applya voltage to biological cells in proximity thereof or attached thereto.

Nanometer-sized or micrometer-sized semiconductor structures can beemployed in a variety of applications, such as light-emitting devicesand photodetectors. Despite the recent rapid developments in designingnovel nanometer-sized and micrometer-sized quantum structures, andincorporating them into a variety of systems, a need still exists forimproved nanometer-sized and micrometer-sizes structures that canreliably perform selected functions in response to specific stimuli.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a nanophotovoltaic device,herein also referred to as an active nanoparticle or simply ananoparticle, that includes a semiconductor structure and a metalliclayer disposed on at least a portion of the semiconductor structure toform a junction (e.g., a Schottky barrier junction) therewith such thatthe junction generates a space charge region. The photovoltaic devicecan have a size in a range of about 50 nm to about 5 microns, andpreferably in a range of about 100 nm to 1000 nm.

Exposure of the nanoparticle to radiation having a selected wavelengthcan cause generation of electron-hole pairs therein. The space chargeregion associated with the semiconductor/metal junction supports anelectric field that can cause separation of these electron-hole pairs tofacilitate generation of a voltage across the device.

In another aspect, the semiconductor structure can be suitably dopedsilicon or germanium. For example, the semiconductor structure caninclude p-type or n-type silicon with a doping level in a range of about10¹⁵ to about 10²⁰ cm⁻³. Alternatively, the semiconductor structure caninclude a Group III-V semiconductor compound, such as, GaAs and GaN.Preferably, the semiconductor structure is formed of a biocompatiblematerial.

More generally, semiconductors useful in producing the nanophotovoltaicdevices of the present invention can include Group II-VI, III-V andgroup IV semiconductors. (Alternatively, using the new IUPAC system fornumbering element groups, suitable semiconductor materials include, butnot limited to, the following: materials composed of a first elementselected from Group II of the Periodic Table of the Elements and asecond element selected from Group 2 or 12 of the Periodic Table ofElements and a second element selected from Group 16 (e.g., ZnS, ZnSe,ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS, CaSe,CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and the like); materialscomposed of a first element selected from Group 13 of the Periodic Tableof the Elements and a second element selected from Group 15 (GaN, GaP,GaAs, GaSb, InN, InP, InAs, InSb, AlS, AlP, AlAs, AlSb, and the like);materials composed of a Group 14 element (Ge, Si, and the like); GroupIV-VI materials such as PbS, PbSe and the like; and alloys and mixtures(including ternary and quaternary mixtures) thereof.

In a related aspect, the metallic layer can comprise any suitable metal,and preferably, a metal that is biocompatible. Some examples of metalssuitable for forming the metallic layer include, without limitation,gold, silver, platinum, titanium, palladium, and alloys thereof.

In yet another aspect, the semiconductor structure of the nanoparticlecan exhibit a bandgap with a bandgap energy in a range of about 0.5 eVto about 2 eV. Such a nanoparticle can be exposed to radiation with oneor more wavelength components that substantially correspond to thebandgap energy to excite electrons from the semiconductor's valence bandto its conduction band, thereby generating a plurality of electron-holepairs. Preferably, the activating radiation can include wavelengthcomponents in a range of about 400 nm to about 2000 nm. Preferably, thewavelength components lie in the infrared portion of the electromagneticspectrum, for example, in a range of about 600 nm to about 1100 nm, thatcan pass through a patient's skin and/or other tissue to activatenanoparticles previously administered to the patient, as discussed inmore detail below.

In another aspect, an electrically insulating layer having a thicknessin a range of about 5 angstroms to about 500 angstroms, and morepreferably in a range of about 50 to about 100 angstroms, can coat atleast a portion of the nanoparticle's semiconductor structure. Forexample, when the nanostructure comprises silicon, a thin layer ofsilicon oxide (SiO₂) having a thickness in a range of about 5 to about10 nanometers can coat the circumferential surface of the semiconductorstructure (i.e., the surface extending between the metallic layers) soas to passivate that surface.

In further aspects, a plurality of ligands can be coupled to at least aportion of the nanoparticle's surface, for example, by forming covalentbonds therewith. For example, the ligands can be attached to thenanoparticle's surface via linker compounds previously coupled to thatsurface. The ligands can have affinity for certain biological cells, forexample, certain cancer cells, to allow the associated nanoparticles toattach to those cells. For example, the ligands can include an antibodythat can attach to selected trans-membrane receptor proteins of a celltype of interest, such as, a particular tumor cell, thereby anchoringthe corresponding nanoparticles to these cells.

The nanoparticles are preferably biocompatible and can be injected intoa selected tissue, e.g., cancerous tissue, and activated, for example,by irradiation at a suitable wavelength, to cause generation of avoltage across them. An electric field associated with the inducedvoltage can be experienced by tissue cells in vicinity of thenanoparticles or attached thereto. The applied electric field can besufficiently high so as to disrupt functioning of the cells or causetheir death.

In another aspect, the invention provides a nanophotovoltaic devicehaving a semiconductor structure that comprises a p-n junction formed byadjacent semiconductor p-doped and n-doped portions. The p-n junctioncan provide a space charge region that can facilitate separation ofelectron-hole pairs generated in the nanoparticle, in response toexposure to a radiation having a suitable wavelength, so as to generatea voltage across the nanoparticle. The semiconductor structure can havea size in a range of about 50 nm to about 5 microns, and preferably in arange of about 100 nm to about 1000 nm. The nanoparticle can furtherinclude a pair of metallic layers, each disposed on a portion of thesemiconductor structure to form an ohmic contact therewith. In someembodiments, at least one ligand, which has affinity for cells of aselected type, can be coupled to an external surface of thenanoparticle.

In a related aspect, the semiconductor structure can include, withoutlimitation, silicon, germanium or a Group III-V semiconductor compound,or any of the other semiconductor compounds recited above. Further, eachof the p-doped portion and the n-doped portion can have a doping levelin a range of about 10¹⁵ to about 10²⁰ cm⁻³, and more preferably in arange of about 10¹⁷ to about 10¹⁹ cm⁻³.

In a related aspect, the metallic layers can be formed of any suitablemetal, such as, gold, silver, platinum, titanium, palladium, tungsten oralloys thereof. Further, the metallic layers can have a thickness in arange of about 100 angstroms to about 1 micron, and more preferably in arange of about 100 angstroms to about 500 angstroms.

An electrically insulating coating having a thickness in a range ofabout 5 to about 10 nm, can cover at least a portion of an externalsurface of the nanoparticle. For example, the insulating layer canextend from one metallic layer to the other so as to coat an externalcircumferential surface of the nanoparticle, or a portion thereof. Forexample, when the semiconductor structure comprises silicon, a coatingof silicon oxide can cover at least a portion of its circumferentialsurface. The nanoparticle can be irradiated with radiation having awavelength that substantially corresponds to a bandgap of thesemiconductor portion so as to generate electron-hole pairs. Withoutlimitation, some suitable wavelengths can lie in a range of about 400 nmto about 2000 nm, or preferably in a range of about 600 nm to about 1100nm.

In yet another aspect, the invention provides a semiconductornanoparticle that includes an n-doped semiconductor portion having alargest dimension in a range of about 50 nm to about 5 microns, and ap-doped semiconductor portion, also having a largest dimension in arange of about 50 nanometers to about 5 microns, that is disposedadjacent to the n-doped portion so as to generate a p-n junctiontherewith. The n-doped and the p-doped portions can be formed ofsilicon, germanium, or a any other suitable semiconductor compound, suchas those recited above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A presents a schematic cross-sectional view of a nanophotovoltaicdevice according to one embodiment of the invention having a Schottkybarrier junction,

FIG. 1B illustrates a perspective view of the nanophotovoltaic deviceshown in FIG. 1A,

FIG. 2A is a schematic perspective view of a nanophotovoltaic deviceaccording to another embodiment of the invention having a p-n junctionsandwiched between two ohmic metallic layers,

FIG. 2B is a schematic cross-sectional view of the nanophotovoltaicdevice of FIG. 2A,

FIG. 3 is a flow chart depicting various steps in exemplary methodsaccording to one embodiment of the invention for fabricatingnanophotovoltaic devices,

FIG. 4A schematically depicts a semiconductor-on-insulator (e.g., SIMOX)wafer that can be utilized for fabricating nanophotovoltaic devicesaccording to the teachings of the invention,

FIG. 4B schematically depicts a p-n junction formed in an upper siliconlayer of the SIMOX wafer of FIG. 4 during one of the processing stepsdepicted in the flow chart of FIG. 3,

FIG. 4C schematically depicts a thin metallic layer formed over an uppersilicon layer of the SIMOX wafer shown in FIG. 4A to form a Schottkybarrier junction or an ohmic contact layer with the underlyingsemiconductor layer,

FIG. 5A schematically depicts the wafer shown in FIG. 4C mounted upsidedown to a support wafer via the thin metallic layer by employing adissolvable adhesive,

FIG. 5B schematically depicts the wafer of FIG. 5A with its bulk siliconportion removed during a processing step listed in the flow chart ofFIG. 3,

FIG. 6A schematically depicts the wafer of FIG. 5B with the continuousburied oxide layer of the original SIMOX wafer removed during aprocessing step listed in the flow chart of FIG. 3 to expose a backsurface of the SIMOX wafer's upper silicon layer,

FIG. 6B schematically depicts the semiconductor structure of FIG. 6Awith a thin metallic layer deposited over the exposed surface of theoriginal SIMOX wafer's upper silicon layer to form an ohmic contacttherewith,

FIG. 7A schematically depicts a relief layer disposed over the exposedmetallic layer of the semiconductor structure of FIG. 6B during aprocessing step in the flow chart of FIG. 3 to provide exposed andunexposed portions of the surface underlying the relief layer,

FIG. 7B schematically depicts a semiconductor structure generated byetching away the portions of the semiconductor structure of FIG. 7A inregister with the openings of the relief layer during one processingstep in the flow chart of FIG. 3,

FIG. 8A schematically depicts the semiconductor structure of FIG. 7Bwith the resist portions removed as a plurality of nanophotovoltaicdevices anchored to the support wafer,

FIG. 8B schematically depicts a plurality of individual nanophotovoltaicdevices according to the teachings of the invention generated byreleasing the devices of FIG. 8A from the support wafer,

FIG. 9 schematically illustrates a nanophotovoltaic device according tothe teachings of the invention electrically coupled to an external loadfor supplying a current thereto upon activation by electromagneticradiation having selected wavelength components,

FIG. 10 is a schematic cross-sectional view of a nanophotovoltaic deviceaccording to one embodiment of the invention that includes a pluralityof ligands attached to a portion of its outer surface, and

FIG. 11 schematically depicts the nanophotovoltaic device of FIG. 10attached to a cell via its ligands.

DETAILED DESCRIPTION

The present invention is directed generally to nanophotovoltaic devices,herein also referred to as active nanoparticles or simply nanoparticles,that can exhibit a desired effect in response to activation. Forexample, a voltage can be induced across the nanoparticles byirradiating them with radiation having suitable wavelength components.For example, radiation with wavelength components in a range of about400 nm to about 2000 nm can be employed. In some embodiments, thenanoparticles include ligands that allow them to couple to a cell typeof interest. The attached nanoparticles can be activated to apply avoltage across the cells to which they are anchored so as to cause adesired therapeutic effect.

The nanoparticles described in the following embodiments of theinvention can be classified broadly as belonging to two categories. Inone category, the nanoparticles include a semiconductor core and twometallic layers disposed on selected portions of the core such that onemetallic layer forms a Schottky barrier junction with the underlyingsemiconductor core and the other forms an ohmic contact therewith. Inthe other category, the nanoparticles include a semiconductor corecomprising a p-n junction and two metallic layers that are disposed onthe semiconductor core to form ohmic contacts therewith. In both cases,the junctions (a Schottky barrier junction or p-n junction) can becharacterized by an internal electric field in proximity thereof thatcan facilitate separation of electron-hole pairs generated in responseto exposure of the core to activating radiation having suitablewavelength components.

By way of example, FIGS. 1A and 1B schematically depicts ananophotovoltaic device 10 according to one embodiment of the inventionthat includes a core 12 formed of a suitable semiconductor—preferably abiocompatible semiconductor material. In some embodiments, thesemiconductor core is formed of silicon or germanium. In otherembodiments, the semiconductor core is formed of any suitable GroupIII-V semiconductors. More generally, semiconductors useful in producingthe nanophotovoltaic devices of the present invention can include GroupII-VI, III-V and group IV semiconductors. (Alternatively, using the newIUPAC system for numbering element groups, suitable semiconductormaterials include, but not limited to, the following: materials composedof a first element selected from Group 2 or 12 of the Periodic Table ofthe Elements and a second element selected from Group 16 (e.g., ZnS,ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, MgS, MgSe, MgTe, CaS,CaSe, CaTe, SrS, SrSe, SrTe, BaS, BaSe, BaTe, and the like); materialscomposed of a first element selected from Group 13 of the Periodic Tableof the Elements and a second element selected from Group 15 (GaN, GaP,GaAs, GaSb, InN, InP, InAs, InSb, AlS, AlP, AlAs, AlSb, and the like);materials composed of a Group 14 element (Ge, Si, and the like); GroupIV-VI materials such as PbS, PbSe and the like; and alloys and mixtures(including ternary and quaternary mixtures) thereof.

The semiconductor core 12 can have p-type or n-type doping with a dopinglevel in a range of about 10¹⁵ to about 10²⁰ cm⁻³, and more preferablyin a range of about 10¹⁷ to about 10¹⁹ cm⁻³. For example, the core canbe formed of n-type silicon (e.g., silicon doped with phosphorous) orp-type silicon (e.g., silicon doped with boron). The semiconductor corecan have any suitable shape, such as, cylindrical, cubic, box-like, orspherical.

In this embodiment, the semiconductor core has a cylindrical shapehaving two opposed, and substantially flat, surfaces 14 and 16 that areseparated by a curved circumferential surface 18. Two metallic layers 20and 22 are disposed, respectively, on the surfaces 14 and 16 such thatone of the metallic layers, e.g., the layer 20, forms a barrier junctioncharacterized by a space charge region (e.g., a Schottky barrierjunction) with the underlying semiconductor surface, and the othermetallic layer, e.g., layer 22, forms an ohmic contact with itsrespective underlying semiconductor surface. The metallic layers, whichare preferably biocompatible, can include, for example, gold (Au),silver (Ag), platinum (Pt), titanium (Ti) and palladium-gold (Pd—Au)alloy.

As discussed in more detail below, the space charge region associatedwith the barrier junction can facilitate separation of electron-holepairs that can be generated by exposing the nanoparticle to radiationhaving selected wavelength components.

In this embodiment, a protective insulating layer 24, which can have athickness in a range of about 5 nm to about 10 nm, circumferentiallysurrounds the core 12. For example, when the core 12 is formed ofsilicon, the protective oxide layer can be SiO₂.

The exemplary nanophotovoltaic device 10 can have a height H in a rangeof about 50 nm to about 5 microns, and more preferably in a range ofabout 100 nm to about 1 micron, and a diameter D in a range of about 50nm to about 5 microns, and more preferably in a range of about 100 nm toabout 1 micron.

As noted above, another class of nanophotovoltaic devices according tothe teachings of the invention include a semiconductor portion thatcomprises a p-n junction. For example, FIGS. 2A and 2B schematicallyillustrate a perspective view and a cross-sectional view, respectively,of such a nanophotovoltaic device 26 in accordance with anotherembodiment of the invention that includes a semiconductor structure 28comprising an n-doped semiconductor portion 28 a that forms a p-njunction with a p-doped semiconductor portion 28 b. The semiconductorstructure 28 is sandwiched between two metallic layers 30 and 32, eachof which forms an ohmic contact with an underlying semiconductorsurface. The semiconductor structure can have a height H in a range ofabout 50 nm to about 5 microns, and preferably in a range of about 100nm to about 1 micron, and a diameter D in a range of about 50 nm toabout 5 microns, and preferably in a range of about 100 nm to about 1micron. Further, each metallic layer can have a thickness in a range ofabout 100 angstroms to about 1 micron.

The semiconductor portions can be formed, for example, of silicon (Si),germanium (Ge), a Group III-V semiconductor compound or any suitablesemiconductor material, such as those listed above. For example, then-doped portion can be formed of silicon that is doped with a donor(e.g., phosphorus) having a concentration, for example, in a range ofabout 10¹⁵ to about 10²⁰, and preferably in a range of about 10¹⁷ toabout 10¹⁹ cm⁻³. And the p-doped portion can be formed of silicon thatis doped with an acceptor (e.g., boron) having a concentration, forexample, in a range of about 10¹⁵ to about 10²⁰ cm⁻³, and preferably ina range of about 10¹⁷ to about 10¹⁹ cm⁻³. The metallic layers can beformed of any suitable metal—preferably biocompatible—that can generatea reliable ohmic contact with the underlying semiconductor surface. Forexample, titanium (Ti), palladium (Pd), gold (Au), silver (Ag) or alloysthereof (e.g., Ti—Pd—Au alloy) can be employed for forming the metalliclayers.

Similar to the previous embodiment, a passivating insulating layer 34,e.g., a layer of SiO₂, can circumferentially surround the semiconductorcore structure 28. This coating layer can have a thickness in a range ofabout 5 angstroms to about 500 angstroms, or preferably in a range ofabout 50 angstroms to about 100 angstroms.

With reference to a flow chart 36 of FIG. 3 and schematic diagrams ofFIGS. 4A-8B, in exemplary methods according to the teachings of theinvention for fabricating the above nanoparticle 10 having a Schottkybarrier junction and the above nanoparticle 26 having a p-n junction, inan initial step 1, a semiconductor-on-insulator (e.g., SIMOX) wafer 38,shown schematically in FIG. 4A, is procured. Asemiconductor-on-insulator wafer can include a semiconductor waferhaving a buried insulating layer that separates an upper portion fromthe bulk of the wafer. For example, as is known in the art, the SIMOXwafer 38 comprises a silicon substrate 40 in which a continuous buriedoxide layer 42 that functions as an electrically insulating layer isformed to separate an upper silicon segment 40 a from the bulk of thesubstrate 40 b. Such SIMOX and bonded silicon-on-insulator wafers arewell known in the art and are commercially available.

For fabricating a nanoparticle having a p-n junction, in step 2, a p-njunction is formed in the upper silicon portion 40 a of the SIMOX waferby utilizing known techniques, such as ion implantation, or knownepitaxial growth techniques. For example, donor ions (such asphosphorous) can be implanted in the upper silicon portion of a p-dopedSIMOX wafer to generate an n-doped layer 44 adjacent a p-doped layer 46within a section of the upper silicon layer, as shown schematically inFIG. 4B. The energy of the ions can be selected in a manner known in theart to ensure that the ion deposition peak lies in a selected region ofthe upper segment, thereby generating the n-doped portion 44 below thep-doped portion 46. When the initial wafer is selected to includeacceptor ions (p-type silicon), the dosage of the implanted donor ionsin a region to be rendered n-doped is selected to be sufficiently highso as to generate donor states having a concentration that is higherthan that of previously present acceptor states.

With continued reference to the flow chart 36, in step 3, a thinmetallic layer 48, having a thickness in a range of about 100 angstromsto about 1 micron, and more preferably in a range of about 100 angstromsto about 500 angstroms is deposited over a top surface of the siliconsegment comprising the p-n junction to form an ohmic contact layertherewith, as shown schematically in FIG. 4C. The metallic layer ispreferably biocompatible, for example, gold, silver, platinum, or anyother suitable metal, and can be formed, for example, by sputtering orevaporating a selected metal in a manner known in the art onto theunderlying semiconductor surface, or by employing any other suitabletechnique. In some embodiments, the metallic layer 48 includes amulti-layer structure. For example, initially a thin layer, e.g., 50angstroms, of titanium is deposited over the semiconductor surface toprovide enhanced adhesion of a subsequent gold layer, e.g., about 100angstroms thick, deposited over the titanium layer.

Referring again to the flow chart 36, for fabricating nanoparticleshaving Schottky barrier junctions, in step 4, the metallic layer 48(FIG. 4C) deposited over a top surface of the SIMOX wafer is selected soas to generate a Schottky barrier junction between the metallic layerand the underlying silicon surface. Any suitable metal—preferablybiocompatible—that is capable of forming a Schottky barrier junctionwith the underlying semiconductor layer can be employed. In manyembodiments, metals having Schottky barrier heights that are about ⅔ ofthe bandgap of the underlying semiconductor are employed. The metallicSchottky barrier layer can have a thickness in a range of about 100angstroms to about 1 micron, or preferably in a range of about 100angstroms to about 500 angstroms.

As fabrication of nanoparticles having p-n junctions and those havingSchottky barrier junctions have the following remaining processing stepsin common, these steps will be described below without regard to thepresence or absence of a p-n junction in the upper silicon segment ofthe SIMOX wafer.

More specifically, referring again to the flow chart 36, subsequent todeposition of the thin metallic layer to form an ohmic contact or aSchottky barrier junction, in step 5, the silicon substrate is mountedupside down, via the deposited metallic layer, on a silicon supportwafer 50 by employing a dissolvable adhesive layer 50 a, such as epoxy,as shown schematically in FIG. 5. Subsequently, in step 6, the bulksilicon portion 40 b of the SIMOX wafer is removed, for example, bylapping and selective etching until the silicon dioxide layer, which canbe employed as the etch stop, is reached, as shown schematically in FIG.5B.

Subsequently, in step 7, the silicon dioxide layer can be etched away byemploying dry etching techniques or other suitable techniques, such asetching in BHF, so as to expose a surface 52 of the upper siliconportion 40, as shown schematically in FIG. 6A. This is followed bydepositing a thin metal layer 54 (shown schematically in FIG. 6B) overthe exposed portion to form an ohmic contact therewith. The metalliclayer can have a thickness in a range of about 100 angnstroms to about 1micron.

Subsequently, in step 9, a relief pattern 56, shown schematically inFIG. 7A, providing exposed and unexposed portions of the underlyingsurface can be formed over the metallic layer 54 by depositing aphotoresist layer over the metallic surface, holographically patterningthe resist, and developing the pattern. More specifically, a photoresistcan be spin-cast over the surface and exposed to a holographic linegrating, rotated by 90 degrees, and exposed again. Alternatively, asingle exposure to a two-dimensional holographic pattern can beemployed. The exposed photoresist can then be developed in a mannerknown in the art to generate the relief pattern 56. The use ofholographic lithography in generating the relief pattern can beadvantageous as it allows obtaining a relief pattern with a resolution,e.g., a 1000 nm, commensurate with the size of nanoparticles produced insubsequent processing steps, as discussed below.

In step 10, the exposed portions of the semiconductor structure and themetallic layers, i.e., the portions not masked by the relief pattern canbe, removed, e.g., via etching, as shown schematically in FIG. 7B. Forexample, a dry etching process, such as those known in the art, can beemployed to etch away the exposed portions of the thin ohmic metalliclayer 54, the underlying thin silicon layer 40 a, and the thin ohmic (orSchottky) metallic layer 48. The etching process can be terminated uponreaching the adhesive layer 50 a or the support wafer 50.

Subsequently, the remaining portions of the resist layer can be removed(step 11), for example, by dissolution in an appropriate solvent, togenerate individual nanophotovoltaic devices (or nanoparticles) 58anchored to the support wafer, as shown schematically in FIG. 8A. Thisis followed in step 12 by releasing the nanoparticles from the supportwafer 50 by dissolving the adhesive in a suitable solvent.

In some embodiments of the invention, the nanoparticles are thenoxidized to form a circumferential oxide layer, having a thickness in arange of about 5 angstroms to about 50 nm, or preferably in a range ofabout 5 nm to about 10 nm, that passivates the particles' exposedsemiconductor surfaces. For example, in the present embodiment, theparticles released from the support wafer can be placed in an oxidizingsolution, for example, a peroxide solution, to cause a portion, andpreferably substantially all, of the exposed circumferentialsemiconductor surface to oxidize, thereby forming a silicon oxide (SiO₂)layer. Alternatively, the nanoparticles can be exposed, e.g., whileattached to the support 50, to a high temperature oxidizing environmentto cause their exposed semiconductor surfaces to oxidize.

The nanophotovoltaic devices formed according to the above embodiment ofthe invention, such as the above nanophotovoltaic devices 10 and 26, canfind a variety of applications. For example, as shown schematically inFIG. 9, the nanophotovoltaic devices can be utilized to generate anelectrical current in a load 60, in response to exposure to radiation 62having suitable wavelength components. For example, a nanophotovoltaicdevice 64 according to the teachings of the invention can be coupled inseries with the load 60 by electrically connecting its metallic layers64 a and 64 b to terminals A and B of the load, respectively.Electron-hole pairs can be created in the particle's semiconductor coreby exposing it to radiation having suitable wavelength components, e.g.,wavelengths corresponding to energies that substantially match, or aregreater than, the semiconductor bandgap energy. The space charge regionin proximity of a junction within the semiconductor core, e.g., a p-njunction, or between the semiconductor core and one of the metalliclayers, e.g., a Schottky barrier junction, facilitates separation ofthese electron-hole pairs so as to generate a voltage across theparticle, and hence a current through the external load 60. For example,electron-hole pairs can be generated in a semiconductor core formed ofsilicon that exhibits, for example, a direct bandgap energy of about1.12 eV at 300 K by irradiating it with radiation having wavelengthscommensurate with the bandgap.

In one application, the nanophotovoltaic devices of the invention, whichcan be biocompatible, can be injected into a diseased tissue, e.g.,cancerous tissue, and be activated, via suitable radiation, to generateelectric fields within that tissue for causing disruption of functioningof the tissue cells. In such applications, the activating radiation ispreferably selected to penetrate the tissue. For example, radiation withwavelength components in a range of about 600 nm to about 1100 nm can beemployed.

In another application, the nanophotovoltaic devices of the inventioncan be functionalized to attach to particular cell types to perform avariety of therapeutic actions. For example, in one embodiment, linkingreactive groups can be attached to the particles' external surfaces tofacilitate their coupling to a particular type of cancer cells in ahuman patient. The functionalized particles can be introduced into thepatient to seek out and attach to the cancer cells. An externalradiation source can then be utilized to irradiate the particles withradiation that passes through the patient's skin and can also generateelectron-hole pairs in the nanoparticles. For example, radiation withwavelength components in a range of about 400 nm to 2000 nm, andpreferably in a range of 600 nm to about 1100 nm, can be employed forthis purpose. The space charge regions associated with the p-n orSchottky barrier junctions of the nanoparticles can facilitateseparation of the electron-hole pairs, thereby generating a voltageacross the particle that is applied to an attached cell. This appliedvoltage can disrupt functioning of the cell or cause its death.

By the example, FIG. 10 schematically illustrates a nanoparticle 66formed in accordance with one embodiment of the invention that, similarto the above nanoparticles 10 or 26, includes a semiconductor core 66 asurrounded by a thin oxide layer 66 b, and two metallic layers 66 c and66 d disposed on two opposed surfaces of the semiconductor core. Aplurality of ligands 68 are attached to the particle's external surface,for example, its circumferential oxide layer and/or its metallic layers.

As shown schematically in FIG. 11, the exemplary ligands 68 facilitatecoupling of the nanoparticle 66 to a cell 70, for example, a cancercell. Illumination of the nanoparticle by radiation, for example,radiation in the infrared region of the electromagnetic spectrum, cancause generation of a voltage across the particle, and consequentlyacross the cell to which the nanoparticle is attached. In this manner, avoltage can be applied to the cell so as to cause disruption of thecell's activity, or to cause its death.

A variety of ligands can be attached to the nano-particles, and avariety of techniques can be utilized for their attachment. While insome embodiments ligands of interest are directly coupled to a portionof the particle's surface, in other embodiments the ligands can becoupled to the particle's surface via linker molecules providing abiocompatible coating of the surface. Further, in some embodiments, atleast a portion of the particle's external surface, e.g., thecircumferential portion, can be activated to facilitate coupling of thelinker molecules and/or the ligands thereto. Such surface activation canlead to modification of one or more surface characteristics, e.g., itmay render the surface more hydrophilic or more hydrophobic, so as tofacilitate its subsequent functionalization. One example of surfaceactivation includes forming an oxide layer over at least a part of theparticle's semiconductor circumferential surface. Exemplary methods forforming such an oxide layer were described above. Other suitable methodsof surface activation, such as, exposure to an ECR plasma or ionimplantation, are described in a commonly owned co-pending patentapplication entitled “Surface Activation of Semiconductor Nanostructuresfor Biological Applications,” which is herein incorporated by referencein its entirety.

The coupling of the biological ligands and/or the linker molecules to ananophotovoltaic device formed in accordance with the teachings of theinvention can be achieved, for example, via formation of a covalent or anon-covalent bond (e.g., an ionic bond) as well as van der Wallsinteractions, or other interactions known in the art.

Some exemplary biological ligands suitable for coupling to photovoltaicparticles formed in accordance with the teachings of the invention caninclude, without limitation, proteins, peptides, nucleic acids,polysaccarides, antibodies or antibody fragments, and antigens. In someembodiments, the nanoparticles are immersed in a solution containing aselected quantity of ligand molecules of interest so as to causecoupling of the ligand molecules to the particles' surfaces.

In some embodiments of the invention, the ligands coupled to thephotovoltaic particles include antibodies, or antibody fragments, thatcan selectively attach to a cell type of interest. By way of example,such antibodies, or antibody fragments and constructs, can targettumor-associated antigens of a particular cancer type.

Those having ordinary skill in the art will appreciate that variousmodifications can be made to the above embodiments without departingfrom the scope of the invention.

1. A nanophotovoltaic device, comprising a three-dimensional semiconductor structure comprising a p-doped portion and an n-doped portion, which form a p-n junction, said semiconductor structure having a size in each of said three dimensions in a range of about 50 nm to about 5000 nm, a pair of metallic layers, each disposed on a portion of said semiconductor structure to form an ohmic contact therewith, wherein exposure of said semiconductor structure to radiation having at least one selected wavelength causes creation of electron-hole pairs separated by a space charge generated by said p-n junction to provide a voltage across said metallic layers.
 2. The nanophotovoltaic device of claim 1, wherein said metallic layers have a thickness in a range of about 100 angstroms to about 1 micron.
 3. The nanophotovoltaic device of claim 1, wherein said semiconductor structure comprises any of Si and Ge.
 4. The nanophotovoltaic device of claim 1, wherein said semiconductor structure is selected from the group consisting of Group II-VI, Group III-V and Group IV semiconductors.
 5. The nanophotovoltaic device of claim 1, wherein said n-doped portion has a doping in a range of about 10¹⁵ cm⁻³ to about 10²⁰ cm⁻³.
 6. The nanophotovoltaic device of claim 1, wherein said p-doped portion has a doping in a range of about 10¹⁵ cm⁻³ to about 10²⁰ cm⁻³.
 7. The nanophotovoltaic device of claim 1, further comprising an insulating coating covering at least a portion of an external surface of said device.
 8. The nanophotovoltaic device of claim 7, wherein said insulating layer extends between said metallic layers.
 9. The nanophotovoltaic device of claim 7, wherein said insulating layer comprises an oxide layer.
 10. The nanophotovoltaic device of claim 7, wherein said insulating layer has a thickness in a range of about 5 nm to about 10 nm.
 11. The nanophotovoltaic device of claim 1, wherein said at least one selected wavelength lies in a range of about 400 nm to about 2000 nm.
 12. The nanophotovoltaic device of claim 11, wherein said at least one selected wavelength lies in a range of about 600 nm to about 1100 nm.
 13. The nanophotovoltaic device of claim 1, further comprising a ligand attached to a portion of said device.
 14. A nanophotovoltaic device, comprising a three-dimensional n-doped semiconductor portion having a size in each of said three dimensions in a range of about 50 nm to about 5000 nm, and a three-dimensional p-doped semiconductor portion disposed adjacent to said n-doped portion to generate a p/n junction, said p-doped portion having a size in each of said three dimensions in a range of about 50 nm about 5000 nm.
 15. The nanophotovoltaic device of claim 14, wherein said n-doped portion comprises any of silicon, germanium, or a Group III-V semiconducting composition.
 16. The nanophotovoltaic device of claim 14, wherein said p-doped portion comprises any of silicon, germanium, or a Group III-V semiconducting composition.
 17. The nanophotovoltaic device of claim 14, wherein said p-n junction provides a space charge region for facilitating separation of electron-hole pairs generated in response to irradiation of said p-n junction with radiation having one or more selected wavelength components.
 18. The nanophotovoltaic device claim 17, wherein said wavelength components lie in a range of about 400 nm to about 2000 nm.
 19. The nanophotovoltaic device of claim 17, wherein said wavelength components lie in a range of about 600 nm to about 1100 nm.
 20. The nanophotovoltaic device of claim 17, further comprising a pair of metallic layers disposed on selected portions of an external surface of said p-n junction to extracting a voltage generated by said separated electron-hole pairs. 